28 October 2021
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PU’s achilles heel

Image: stock.adobe.com/Sved Oliver

Time may soon be called on PU structural adhesives for many rail interior applications. However, Henkel is ready with an alternative, reports Ruairi O’Hare, head global strategy aviation, space & rail

Adhesive bonding has become a key component in the manufacture of parts that offer superior performance and durability in rail vehicles and infrastructure applications. For decades, these objectives were achieved in part with adhesive solutions containing diisocyanates, a family of chemical building blocks mainly used to make polyurethane products.

Polyurethanes were preferred over other chemistries for their high initial tack, exceptional tensile strength, elongation (up to 600%), and high fatigue resistance. They also show effective resistance to rail cleaners, with no loss of mechanical properties, and do reasonably well in UV exposure for exterior applications.

But the use of these compounds will one day be restricted in Europe – and eventually elsewhere due to the Registration, Evaluation, Authorisation and Restriction of Chemicals initiative, or REACH.

REACH calls for the progressive substitution of specific chemicals such as diisocyanates (referred to as “substances of very high concern” or SVHCs) when suitable alternatives have been identified. Having entered into force in 2007, REACH provisions are being phased in over 11 years.

To determine REACH compliance, companies must identify and manage the risks linked to the substances they manufacture or market to the European Union. They have to demonstrate to ECHA (European Chemicals Agency) how they’ve determined a substance can be safely used without harming the end customer.


While the current aim of the legislation is not to ban the use of diisocyanates, but to improve control over their use, the REACH restriction poses a challenge for rail industry OEMs. In addition, many individual local governments/plants/customers are choosing to ban them based on the negative health effects, even though this has not been mandated.

Diisocyanates, in combination with various polyols, are essentially the ‘backbone’ of a polyurethane material, making them almost impossible to eliminate. As a result, rather than cope with regulations that lay down specific migration limits (the maximum permitted quantities of a substance classified as hazardous to health), OEMs are seeking alternative solutions to polyurethanes that will maintain their manufacturing standards in safety, durability and flexibility.

Toward that end many are turning to Henkel (01442 278000), and its global network of engineering and R&D centres staffed by over 3,000 design and application professionals. Henkel’s Silane Modified Polymers (SMPs) are structural adhesives for applications needing high elasticity, primerless adhesion on various substrates and high fatigue resistance.

Like polyurethane adhesives, they were primarily designed and developed to replace welds, rivets and other mechanical fasteners, enabling the assembly of non-metallic substrates with the added benefit of contributing to lighter-weight designs. However, they contain no solvent, isocyanates or silicone, and thus are not restricted by REACH.

Applications for Henkel SMPs include structural bonding of panels, walls, roofs and floors, as well as bonding windscreens and side windows to the train body. These materials eliminate the need for rubber seals, resulting in improved vehicle appearance, and are superior to polyurethanes in outdoor applications where UV resistance is required.

Henkel SMP materials have an advantage in process robustness. Polyurethanes work well, but require an exacting process of cleaning and priming steps that have very specific time limitations. For best results, SMPs require the cleaning and priming, but the process is less sensitive and tolerant of variations. In some cases, primerless adhesion is possible.

Henkel SMP materials are compatible with most paint systems, to allow seamless blending with the exterior finish for enhanced vehicle aesthetics. These solutions provide excellent anti-flutter qualities when bonding inner to outer panels and UV resistance.

Assembly line adjustment would be minimal for this transition, as SMPs can be dispensed by hand or automatically using a manufacturer’s current equipment.

Ruairi O'Hare

Related Companies

Henkel Ltd

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